A minute later, the first character rushes onto the balcony. In this case, which of the above scene headers would be more appropriate? (It's a non- nterrupted movement from the lving room to the balcony)

For a scene that takes place in a house, you would start with a full heading.

INT. HOUSE - DAY

and then if your character moved into the kitchen, then the sub-header could be

KITCHEN

You could do this for any room in the house, but if your character exits the interior of the house, return to using a full slugline. For example:

INT. TRIP'S HOUSE - DAY

Trip stares at his computer screen. His fingers peck away at the keyboard. There's a knock at the back door.

KITCHEN

In the middle of the back door is a small glass window. A young woman peeks through.

Trip opens the door.

TRIPWhere's your key?

YOUNG WOMANI lost it.

BEDROOM

The young woman pulls a sheet off a bed. Trip tosses a key on the dresser.

TRIPDon't lose it.

EXT. VERANDA - DAY

A hammock swings in a light breeze. A key lies beneath it.

TRIPKids.

Clarity rules! So, if there is a hint of a lack of clarity for the reader, default to using a full slugline.

I do not remember where I picked up the quote below but you may find it of interest.

“Remember, a slug is not just a storytelling device, it’s also a production management tool. Slugs not only let a reader know where scenes takes place, but also let the production team know what locations or settings are required for these scenes.”

Lastly, the advice above is offered only as a guide and not a rigid rule. It is only an attempt to answer your question about sluglines and sub-headings within the same location.

I chose balcony door for the first case: Unless mistaken, I think a window, or a door etc is needed, to see both locations at the same time.For the non-interrupted transition from one location to the other I went with the second scene header.